Vaporization and cracker cell method and apparatus

ABSTRACT

A method and apparatus for vaporizing and cracking chemical elements for use in a deposition process. The apparatus includes a vaporization cell integrally connected with a thermal cracker cell. The vaporization cell has an inlet section in communication with a valve section defining a heating chamber capable of holding a liquid or solid chemical material to be vaporized. A heat source is positioned in the heating chamber and is capable of providing sufficient thermal energy to evaporate or sublimate the chemical material. The thermal cracker cell is communicatively connected to an outlet of the vaporization cell, and includes an elongated tapered tube with a heating element associated therewith. The heating element is capable of providing sufficient thermal energy to dissociate molecular clusters of vaporized chemical material. This provides monomeric or dimeric chemical elements for use in a deposition process such as during semiconductor device fabrication.

This is a divisional application of U.S. patent application Ser. No.09/241,805, filed on Feb. 2, 1999, now U.S. Pat. No. 6,447,734 B1, whichis incorporated herein by reference.

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of Grant No.ECS-9502891 awarded by the National Science Foundation (NSF).

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention relates generally to an apparatus and method fortransforming chemical elements from a solid state to a gaseous state.More specifically, the present invention relates to an apparatus andmethod for vaporizing and cracking chemical elements for use in adeposition process such as during semiconductor device fabrication.

2. The Relevant Technology

Various chemical elements are used in conventional deposition processesperformed during semiconductor device fabrication. For example, thechemical elements in Group V of the periodic table, such as phosphorusand arsenic, are commonly used as dopants in semiconductor processingtechnologies and are vital materials in several semiconductor devices.It is desirable to be able to convert the solid forms of these chemicalelements into a form which may be subsequently combined with otherchemical elements to create the desired product. To accomplish this, thechemical elements must first be vaporized.

Conventional techniques for converting chemical elements into vaporphase for semiconductor device fabrication employ a vacuum evaporationsystem. Generally, the vacuum evaporation system includes a heating unitand is in communication with a growth chamber for deposition of theelement onto a substrate. The heating unit is used to supply therequired energy to convert the element into a vaporous form, and thegrowth chamber is ideally operated under high vacuum conditions, as thisensures a high quality, non-contaminated crystal. One technique thatemploys such a vacuum evaporation system is molecular-beam epitaxy(MBE).

In MBE, a variety of sources can be employed for flux generation, andtheir design depends on the nature of source materials. The thermaleffusion source or Knudsen-cells (k-cells) are used in nearly all MBEsystems for deposition of semiconductor and/or dopant materials duringsemiconductor device fabrication. A k-cell includes a cruciblecontaining a solid or liquid evaporant, which is radiatively heated byelectrically insulated heater filaments wound around the crucible. Athermocouple, which is carefully positioned to ensure intimate contactwith the crucible, registers the source material temperature and can,via a feed-back loop, control the power to the heater and thus thetemperature of the source. Several layers of refractory metal foil(e.g., tantalum) are wrapped around the entire cell to minimize heatlosses from the cell wall, with the major heat loss being from theeffusion aperture.

Typically, the vaporization process yields varying ratios of chemicalelements in monomeric, dimeric, and tetrameric forms. Conventionalvaporization techniques for group V elements are unable to reduce themajority of the element to a monomeric or dimeric form, resulting in asubstantial amount of tetrameric forms of the element. Such tetramericforms of group V elements are undesirable from the standpoint of use insemiconductor device fabrication. The growth of a crystalline layer,which is required for semiconductor device applications, is bestachieved when the monomeric (atomic) or the dimeric form of the elementis used. Therefore, after vaporization of the chemical element, a methodto efficiently convert clusters of tetrameric forms of the element intomonomeric and dimeric forms is of substantial interest.

Several techniques for converting or “cracking” tetrameric forms ofchemical elements into monomeric or dimeric forms have been developed.Such techniques employ either extremely high temperatures or ultravioletlight to input the energy necessary to separate elemental clusters. Someof these systems, such as the ultraviolet light systems, are verymechanically complex with many small parts requiring continualadjustments in order to achieve optimal performance. For example,precise alignment of parts is necessary to focus an ultraviolet beam ina manner that will achieve efficient cracking of elemental clusters.

The disadvantages of cracking systems that employ extreme heat orultraviolet light include the high maintenance and expense required torun and maintain such systems. The high expense is incurred through boththe power consumption and the mechanical maintenance required. Inaddition, most known systems for evaporating and cracking chemicalelements have separate evaporation and cracker cell units, which reducesthe efficiency of providing the chemical elements in a desirable formfor deposition.

It would therefore be of significant advantage to develop a simple,inexpensive, and efficient system which can perform both the functionsof chemical element vaporization and cracking.

SUMMARY AND OBJECTS OF THE INVENTION

It is an object of the present invention to provide an apparatus forvaporizing and cracking chemical elements for use in a depositionprocess during semiconductor device fabrication.

It is another object of the present invention to provide such avaporizing and cracking apparatus that is simple in design andmanufacture, and inexpensive to use and maintain.

It is another object of the present invention to provide such avaporizing and cracking apparatus which is a fully integrated orcombined effusion system.

It is a further object of the present invention to provide such avaporizing and cracking apparatus which is easy to use and operates atpeak efficiency.

To achieve the foregoing objects, and in accordance with the inventionas embodied and broadly described herein, an apparatus is provided forvaporizing and cracking chemical elements such as group V elements foruse in a deposition process. The apparatus includes a vaporization cellintegrally connected with a thermal cracker cell.

The vaporization cell has an inlet section in communication with a valvesection defining a heating chamber capable of holding a chemicalmaterial to be evaporated or sublimated. A container such as a quartzboat is preferably disposed in the heating chamber for holding thechemical material. A heat source is positioned in the heating chamberand is capable of providing sufficient thermal energy to evaporate orsublimate the chemical material.

The thermal cracker cell is communicatively connected to an outlet ofthe vaporization cell, and includes a tapered elongated tube with aheating element such as a heating coil disposed therearound. The heatingelement is capable of providing sufficient thermal energy to dissociatemolecular clusters of vaporized chemical material. This providesmonomeric or dimeric chemical elements for use in a deposition process.The elongated tube is preferably composed of quartz and has a passagewaywith a diameter of a first dimension that narrows to a smaller seconddimension toward an exit opening of the tube. The tube narrows in orderto cause the gaseous clusters of elements to be separated so that theclusters can receive a greater amount of heat energy as a result ofincreased exposure to and decreased distance from the heating element.This exposure results in greater efficiency in separating elementalclusters, and allows the use of lower temperatures.

In a method for vaporizing and cracking a chemical material whichutilizes the apparatus of the invention, a preselected amount of achemical material is placed into the heating chamber, and the chemicalmaterial is heated to a first temperature sufficient to vaporize thechemical material. The temperature in the heating chamber can bemonitored and adjusted for optimal vaporizing conditions. The vaporizedchemical material is then directed to the elongated tube and is heatedalong the smaller second dimension of the passageway in the elongatedtube to a higher second temperature sufficient to dissociate molecularclusters of vaporized chemical material. The dissociated chemicalmaterial can then be directed from the exit opening of the elongatedtube to a vacuum chamber for deposition on a substrate.

These and other objects, features, and advantages of the presentinvention will become more fully apparent from the following descriptionand appended claims, or may be learned by the practice of the inventionas set forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more fully understand the manner in which the above-recitedand other advantages and objects of the invention are obtained, a moreparticular description of the invention briefly described above will berendered by reference to a specific embodiment thereof which isillustrated in the appended drawings. Understanding that these drawingsdepict only a typical embodiment of the invention and are not thereforeto be considered limiting of its scope, the invention will be describedand explained with additional specificity and detail through the use ofthe accompanying drawings in which:

FIG. 1 is a side view of a vaporization and cracker cell apparatusaccording to the present invention; and

FIG. 2 is a cross-sectional view of the vaporization and cracker cellapparatus of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to an apparatus and method forvaporizing and cracking chemical elements such as group V elements. Theinvention is particularly useful in deposition processes such as thoseemployed during semiconductor device fabrication. The apparatus of theinvention provides an efficient, simple, and economic way of performingboth chemical element vaporization and cracking during a depositionprocess such as MBE.

The apparatus of the invention provides for a two-step heating processin which a chemical material in a solid or liquid state is firstvaporized and then further heated to crack or dissociate vaporousmolecular clusters into monomeric or dimeric forms of a chemicalelement. As used herein, the term “monomeric” refers to the atomic formof any element or a single atom of any element unbound to other atoms ofthe same element, while the term “dimeric” refers to two atoms of anyelement which are bonded to one another.

Referring to the drawings, wherein like structures are provided withlike reference designations, the drawings only show the structuresnecessary to understand the present invention. FIG. 1 illustrates avaporization and cracker cell apparatus 10 according to the presentinvention. The apparatus 10 is shown operatively connected at one end toa power source 12 and a control device 14, which will be discussed infurther detail below in connection with the operation of apparatus 10.The outlet end of apparatus 10 is in communication with a vacuum chamber16 where deposition of a material from apparatus 10 takes place during afabrication process. The apparatus 10 as a whole can be fitted onto anyvacuum chamber by choosing components of appropriate dimension, as fitseach individual chamber, and can operate efficiently in vacuum below themillitorr range.

The apparatus 10 includes two subunits, a vaporization cell 18 and athermal cracker cell 20, which are communicatively attached together inan integral structure. Each of these subunits will be discussed indetail as follows.

As shown in FIGS. 1 and 2, vaporization cell 18 includes an inletsection 22 and an outlet section 24 which are communicatively connectedto a valve section 26 therebetween. The inlet section 22, outlet section24, and valve section 26 are preferably hollow tubular structures whichcan be formed of various materials such as stainless steel. The inletsection 22 and outlet section 24 are preferably attached on oppositesides of valve section 26 in a parallel configuration perpendicular tovalve section 26.

The inlet section 22 has two subsections, including a valve inletportion 28 and a feedthrough section 30. The valve inlet portion 28 iscommunicatively connected at one end to valve section 26 and on theother end has a circular connection flange 32. The feedthrough section30 has a pair of opposing connection flanges 34 and 36 on each endthereof. The connection flange 34 is attached to flange 32 by aplurality of bolts 35 inserted between the flanges to provide forremovable attachment of feedthrough section 30 with valve inlet portion28. The connection flange 36 is removably attached to a feedthroughflange 38 by bolts 35, and the feedthrough flange 38 is removablyattached to a smaller guide flange 39. A sealing gasket 41 such as acopper gasket is preferably disposed between each of the attachedflanges to provide a vacuum seal. The feedthrough flange 38 and guideflange 39 have openings therein permitting insertion of a powerfeedthrough 40 and a thermocouple feedthrough 42 into inlet section 22.As shown in FIG. 1, power feedthrough 40 is operatively connected topower source 12, and thermocouple feedthrough 42 is operativelyconnected to control device 14. The power feedthroughs are individuallyfitted through the flanges to minimize the size constraints.

The valve section 26 of vaporization cell 18 is preferably an in-linemanual valve which includes a tubular section 44 with a circularconnection flange 46 and a valve assembly 48 with a connection flange50. The connection flange 50 is removably attached to connection flange46 by bolts 35 inserted between the flanges. A sealing gasket 51 such asa copper gasket for vacuum seal is disposed between connection flanges46 and 50. The tubular section 44 defines a heating chamber 52, whichholds a chemical element container 54 such as a quartz boat that servesas a crucible for a chemical material 60 to be evaporated or sublimated.As depicted in FIG. 2, container 54 preferably has a handle 56 whichextends into inlet section 22, allowing for easy insertion and removalof container 54 into and from chamber 52. The container 54 is designedto hold a maximum amount of chemical material 60 to be vaporized, insolid or liquid form, without spilling during refill operations.

A heating means is provided such as a heat source 62 for raising thetemperature of chemical material 60 to its vaporization temperature. Theheat source 62 is positioned in chamber 52 adjacent to container 54 andis operatively connected to power feedthrough 40. The heat source 62 canbe provided in the form of a light bulb, a quartz lamp, and the like.For example, a 150 watt light bulb can be placed inside chamber 52directly above chemical material 60 held in container 54. Thethermocouple feedthrough 42, associated with a thermocouple device, isin intimate contact with container 54 through inlet section 22 andregisters the temperature of chemical material 60. The thermocoupledevice can provide, via a feed-back loop, control of the power to heatsource 62 and thus the temperature of chemical material 60.

The valve assembly 48, which can be a bellow valve assembly, includes avalve handle 64 connected to a movable bellow section 66 disposed inchamber 52. The bellow section 66 terminates in a valve seat 70 havingan o-ring 68 for high temperature seal, such as a Calrez™o-ring. Thehandle 64 provides manual control such that below section 66 can beraised and lowered as needed during operation of apparatus 10 in orderto open or close chamber 52 with respect to cracker cell 20.

The outlet section 24 includes an outlet connection flange 72 attachedto a larger circular connection flange 74 by a plurality of bolts 35inserted between the flanges to provide for removable attachment ofcracker cell 20 to vaporization cell 18. A sealing gasket 73 such as acopper gasket is preferably disposed between flanges 72 and 74.

The thermal cracker cell 20 includes a fitting tube 76 and an elongatedheating tube 78 which are in communication with vaporization cell 18.The fitting tube 76 has a tube connection flange 80 that is boltconnected to connection flange 74. A sealing gasket 73 is preferablydisposed between flanges 74 and 80. A passageway 82 within fitting tube76 is in communication with the passageway of outlet section 24. Theheating tube 78 defines a passageway 84 and is removably interconnectedwith fitting tube 76. The fitting tube 76 and heating tube 78 areinterconnected by a plurality of tube fittings 86, 87 and 88, which arepreferably Swage-Lok metallic fittings, or the like, typically used forlow pressure and high temperature applications. The fitting tube 76interconnects with fitting 86, while heating tube 78 interconnects withfitting 88. The fitting tube 76 is preferably made of a metallicmaterial such as stainless steel, while heating tube 78 is preferablymade of a transparent quartz material.

The heating tube 78 defines a first larger diameter for passageway 84which extends into fitting 88 to accentuate the flow of the vapor fromoutlet section 24. The heating tube 78 has a tapering section 89 thatnarrows abruptly such that passageway 84 has a second smaller diameteralong the majority of tube 78 which defines a cracker zone 85 therein.The cracker zone 85 is designed to provide for multiple collisions ofmolecules to maximize the probability of dissociation. The secondsmaller diameter of passageway 84 extends to an exit opening 90 at thedistal end of tube 78 for effusion of the dissociated vaporizedmaterial. In one embodiment, the first larger diameter of passageway 84is about 19 mm and the second smaller diameter is about 4 mm. Thepassageway 82 in fitting tube 76 and the passageway 84 in heating tube78 are aligned for easy and efficient passage of gaseous materials fromvaporization cell 18 to cracker cell 20.

A heating element such as a heating coil 92 is wrapped around theoutside of heating tube 78 toward the distal end thereof along thesmaller diameter portion of passageway 84 defining cracker zone 85. Theheating coil 92 is electrically connected to power source 12 as shown inFIG. 1 by being coupled at a connection junction 94 with a feedthroughconnection 96, which passes through flange 74 for connection with powersource 12. The heating coil 92 can be composed of various metallicmaterials such as refractory metals including tantalum, tungsten,molybdenum, alloys or combinations thereof, and the like. A particularlypreferred material for the heating coil is tantalum.

The heating coil 92 is in close proximity to heating tube 78 andprovides the thermal energy necessary to cleave elemental molecularclusters passing through tube 78. The narrowed portion of passageway 84forming cracker zone 85 in tube 78 has the effect of spacing gaseousmolecular clusters of elements which provides increased exposure of themolecular clusters to the thermal energy provided by heating coil 92.The narrowed portion in tube 78 also has the effect of bringingmolecular clusters in closer proximity to the heat provided by heatingcoil 92. These effects allow for the cracking of molecular clusters in amore efficient manner and at a lower temperature than in prior crackingdevices.

In a method of operating vaporization and cracker cell apparatus 10, apreselected amount of chemical material 60 is placed in container 54,which is inserted into chamber 52 of vaporization cell 18. Theconnection flange 74 is sealingly attached to a conventional connectingflange (not shown) on an inlet of vacuum chamber 16 by bolt connectionssuch that cracker cell 20 is in communication with vacuum chamber 16.The chemical material 60 is then heated sufficiently to vaporize thechemical material. Typically, the temperature in chamber 52 is in arange from about 250° C. to about 400° C., and preferably about 300° C.,in order to vaporize the chemical material. The vaporized material isthen directed into cracker cell 20 and is further heated to a crackingtemperature by heating coil 92 while passing through cracker zone 85 inheating tube 78 to dissociate molecular clusters. The temperature incracker zone 85 is in a range from about 700° C. to about 900° C., andpreferably about 800° C., in order to dissociate the vaporized material.The dissociated molecules can then be directed to vacuum chamber 16 fordeposition on a substrate such as a semiconductor material.

The parameters used in the operation of apparatus 10, such as power,temperature, and pressure, have the capability of being adjusted toachieve the maximum possible yield of the monomeric or dimeric form of achemical element. Power can be controlled externally by power source 12and control device 14 such as a computer or other digital controldevice. The general power consumption for power feedthrough 40 istypically less than about 100 watts for apparatus 10. The temperature ismonitored by the thermocouple device and heat source 62 provides thethermal energy for the vaporization of the chemical material. Theposition of heat source 62 and the amount of power at which it operatesare parameters which may be varied in order to achieve optimization fora given element yield. The range of partial pressures utilized duringoperation of apparatus 10 can be up to about 5×10⁻⁴ torr, and preferablyfrom about 1×10⁻⁵ torr to about 1×1⁻⁴ torr. Optimization runs forapparatus 10 can be carried out with the aid of a residual gas analyzer(RGA) to determine the optimal parameters for a given chemical elementyield.

During operation of apparatus 10, it is preferred that no thermalinteraction occur between vaporization cell 18 and thermal cracker cell20. This prevents loss of control over vaporization when cracker cell 20is operated. The chemical material container 54 generally only needs tobe refilled every few months, depending on the operation frequency ofapparatus 10.

The apparatus of the present invention is particularly effective invaporizing and cracking group V chemical elements such as phosphorus,arsenic, antimony, and combinations thereof. For example, the apparatusof the invention can be used to convert a solid phosphorus source (P₄)such as red phosphorus to its vapor phase, and then to crack thevaporous phosphorus to obtain varying ratios of P₃, P₂, and P. Theapparatus of the invention provides effective control over thegeneration of a vapor such as a phosphor gas. The ratios of the desiredfinal product with respect to P₃, P₂, or P, can be adjusted by varyingthe temperature of the heating coil around the quartz tube, and the rateat which the vapor passes through the quartz tube, depending on therequirement for a given application. The control of the power for thevaporization cell and the cracker cell can provide extremely stablepartial pressures of phosphorus, with almost no noticeable fluctuations.

When arsenic is vaporized and cracked by the apparatus of the invention,it is preferable to provide an additional heat source at various pointsaround apparatus 10 to prevent condensation of the arsenic. For example,a high condensation rate of the arsenic flux can occur at the in-linevalve. The additional heat source can be in the form of conventionalheating tape which can be wrapped around selected outside portions ofapparatus 10.

The apparatus of the present invention provides many advantages andbenefits. The apparatus is a fully integrated or combined effusionsystem of vaporization and cracker cells, has a simple design, and iseasy to manufacture and use. The apparatus provides an effectiveeffusion system which combines the two functions of vaporization andmolecular dissociation. The apparatus operates at peak efficiency, iseasy to assemble and disassemble for maintenance, and is inexpensive tooperate, as compared to prior source cells. The apparatus of theinvention is particularly effective in vaporizing and cracking chemicalelements for use in deposition processes during semiconductor devicefabrication. The apparatus can also be easily upgraded for largerquantities of chemical material without any consequential change indesign.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A method for vaporizing and cracking a chemicalmaterial, comprising: providing a vaporization and cracker cellapparatus comprising: a vaporization cell having a heating chamber forholding a chemical material; a heat source disposed in the heatingchamber; and a thermal cracker cell communicatively connected to anoutlet of the vaporization cell, the cracker cell including an elongatedtube with a heating element associated therewith, the tube having apassageway with a diameter of a first dimension that narrows to asmaller second dimension by way of a tapering section in the tube, thetapering section located distally apart from an inlet of the tube, thesmaller second dimension of the tube being substantially maintained fromthe tapering section to an exit opening of the tube; placing apreselected amount of a chemical material into the heating chamber;heating the chemical material in the heating chamber to a firsttemperature sufficient to vaporize the chemical material; directing thevaporized chemical material to the elongated tube; and heating thevaporized chemical material along the smaller second dimension of thepassageway in the elongated tube to a higher second temperaturesufficient to dissociate molecular clusters of vaporized chemicalmaterial.
 2. The method of claim 1, further comprising directing thedissociated chemical material from the exit opening of the elongatedtube to a vacuum chamber for deposition on a substrate.
 3. The method ofclaim 2, wherein the substrate comprises a semiconductor material. 4.The method of claim 1, wherein the chemical material comprises a solidor liquid material.
 5. The method of claim 1, wherein the chemicalmaterial is selected from the group consisting of phosphorus, arsenic,antimony, and combinations thereof.
 6. The method of claim 1, furthercomprising monitoring the temperature in the heating chamber, andadjusting the temperature in the heating chamber for optimal vaporizingconditions.
 7. The method of claim 1, wherein the elongated tubecomprises a quartz tube.
 8. The method of claim 1, wherein the heatsource is a light bulb or a quartz lamp mounted within the heatingchamber.
 9. The method of claim 1, wherein the vaporization cellincludes a valve section with an in-line valve for selectively sealingthe heating chamber from the passageway in the tube.
 10. The method ofclaim 1, wherein the chemical material is held by a container in theheating chamber.
 11. The method of claim 1, further comprisingmonitoring the temperature in the heating chamber by a thermocoupledevice disposed in the vaporization cell.
 12. The method of claim 1,wherein the heating element of the cracker cell comprises a heating coildisposed around an outside surface of the tube.
 13. A method forvaporizing and cracking a chemical material, comprising: providing avaporization and cracker cell apparatus comprising: a vaporization cellincluding an inlet section in communication with a valve sectiondefining a heating chamber; a quartz container disposed in the heatingchamber for holding a chemical material; a heat source positioned in theheating chamber adjacent to the container; a thermal cracker cellcommunicatively and removably connected to an outlet of the vaporizationcell, the cracker cell including an elongated quartz tube having apassageway with a diameter of a larger first dimension that narrows to asmaller second dimension by way of a tapering section in the tube, thetapering section located distally apart from an inlet of the tube, thesmaller second dimension of the tube being substantially maintained fromthe tapering section to an exit opening of the tube; an in-line valvedisposed in the valve section for selectively sealing the heatingchamber from the passageway in the tube; and a heating coil disposedaround an outside surface of the tube toward the exit opening; placing apreselected amount of a chemical material into the heating chamber;heating the chemical material in the heating chamber to a firsttemperature sufficient to vaporize the chemical material; directing thevaporized chemical material to the elongated tube; and heating thevaporized chemical material along the smaller second dimension of thepassageway in the elongated tube to a higher second temperaturesufficient to dissociate molecular clusters of vaporized chemicalmaterial.
 14. The method of claim 13, wherein the chemical material isselected from the group consisting of phosphorus, arsenic, antimony, andcombinations thereof.
 15. The method of claim 13, further comprisingmonitoring the temperature in the heating chamber by a thermocoupledevice disposed in the vaporization cell.
 16. The method of claim 15,wherein the thermocouple device is operatively connected to a controldevice for monitoring and adjusting the temperature in the heatingchamber.
 17. The method of claim 16, wherein the heat source isoperatively connected to a power source in operative communication withthe control device.
 18. The method of claim 17, wherein the heating coilis electrically connected to the power source.
 19. The method of claim13, wherein the exit opening of the tube is in sealing communicationwith a vacuum chamber.